Marine cyanobacteria are especially well endowed with these types of modular biosynthetic pathways which they employ to produce a rich arsenal of bioactive secondary metabolites. For example, apratoxin A is a depsipeptides showing subnanomolar cytotoxicity to several different cancer cell lines in vitro. Unfortunately, in evaluating apratoxin A for in vivo anticancer activity, it showed limited or no activity against tumors (colon and breast) at sub-lethal doses, and at higher doses, animal toxicity and weight loss were observed.
Apratoxin B and C are simple desmethyl analogs of apratoxin A isolated from collections of Lyngbya sp. Apratoxin D, found in Papua New Guinea, has a carbon skeleton which appears to result from an additional PKS element (as compared to Apratoxins A, B and C) in the biosynthetic pathway. An apratoxin E isolated from a collection of Lyngbya bouillonii made in Guam is an analog with several changes to the decorated structure of apratoxin A.
Apratoxin A has been shown to induce a G1-phase cell cycle arrest and apoptosis via interaction with STAT3 and FGFR signaling; and studies determined that the inhibition of STAT phosphorylation is caused by down regulation of an IL-6 transducer and several cancer specific receptor tyrosine kinases. These alterations resulted in prevention of the translocation of several proteins involved in the secretory pathway and is reversible in normal cells but not in cancer cells. One study showed that an oxazoline analog of apratoxin A interacts with Hsp90, a well known and important cancer cell target.